The present invention is directed generally to permanent magnet assemblies and more particularly to permanent magnet assemblies for MRI systems.
There are various magnetic imaging systems which utilize permanent magnets. These systems include magnetic resonance imaging (MRI), magnetic resonance therapy (MRT) and nuclear magnetic resonance (NMR) systems. MRI systems are used to image a portion of a patient's body. MRT systems are generally smaller and are used to monitor the placement of a surgical instrument inside the patient's body. NMR systems are used to detect a signal from a material being imaged to determine the composition of the material. These systems often utilize two or more permanent magnets directly attached to a support, frequently called a yoke. An imaging volume is provided between the magnets. A person or material is placed into the imaging volume and an image or signal is detected and then processed by a processor, such as a computer.
An MRI magnet should maintain a preset main magnetic field (B0) with a narrow variation because a center frequency of the system is linear to the B0 field and the RF and amplifiers/receivers are tuned to this frequency. The narrow bandwidth of the RF chain defines the allowable variation in the main magnetic field. In a superconducting or resistive MRI magnet, the correct B0 field is achieved by adjusting the current in the magnet coils. In a permanent magnet, the B0 field is determined by the magnetic motive force and the reluctance of the magnet. However, there are often differences between the actual B0 field of a permanent magnet (“PM”) and its designed value due to material property variations and the manufacturing tolerances of the magnet dimensions.
Several methods have proposed and used to adjust the B0 field for permanent MRI magnets. One method includes the use of iron plugs to increase or decrease the air gap in the magnet flux circuit, which in turn decreases or increases the B0 field due to changes in the magnet reluctance. Another method includes changing the operating temperature of the magnet, which changes the magnetization of the permanent magnet poles and thus changes the B0 field. Yet another method includes using resistive coils to adjust the B0 field. However, these methods are often complex to implement and sometimes do not achieve the desired B0 field.